The present invention relates to a single-cylinder circular knitting machine for manufacturing socks and stockings and in particular to a machine with high safety against the accidental escape of the hooks from the platen.
As is known, single-cylinder circular knitting machines generally comprise a needle cylinder which is arranged vertically and on whose skirt a plurality of axial grooves is defined; each one of said grooves slidingly accommodates a needle.
Needle actuation cams are arranged around the needle cylinder and define paths within which a heel of the needles, which protrudes radially from the related groove, engages. The shape of said paths is such as to give, during the rotation of the needle cylinder about its axis with respect to the needle actuation cams, a reciprocating movement to the needles along the related groove. By means of this reciprocating movement, the needles form knitting by cyclically picking up a thread provided at a feed of the machine and forming new loops linked to the previously formed loops of knitting.
Above the needle cylinder there is usually a platen which is arranged coaxially to the needle cylinder and has a plurality of radial grooves, each accommodating a hook.
Each hook is provided with one or more heels which protrude upward from the radial grooves and engage paths defined by cams for the actuation of the hooks which face the platen in an upward position. The actuation of the platen with a rotary motion about its axis, together with the needle cylinder, causes the various hooks to follow these paths, which are shaped so as to move the hooks from a retracted position in the platen to an extraction position in which the hooks protrude radially with their tip from the platen.
The radial grooves of the platen are offset with respect to the axial grooves of the needle cylinder so that each hook can protrude from the platen between two contiguous needles.
The use of the hooks during the operation of these machines is limited to a particular process. In the production of stockings, the use of the hooks is usually limited to the initial step of the process, i.e. during the forming of the upper edge of the stocking, when the hooks are extracted from the platen to form a tubular border in cooperation with the needles. During the forming of the remaining part of the stocking, the hooks generally remain unused and are kept in the platen in a retracted position.
In order to avoid complicating the profile of the hook actuation cams excessively, said cams are limited to a cam which causes the extraction of the hooks and to a cam which causes their retraction, said cams being mutually angularly spaced with respect to the axis of the platen.
For this reason, when the extraction of the hooks is not required, said hooks could protrude accidentally from the platen due to the centrifugal force generated by the rotation of the platen together with the needle cylinder. In order to avoid this unwanted movement, which would cause unwanted alterations in the process in progress, the stem of the hooks is appropriately bent laterally in one or more points of its extension so that by resting against the lateral walls of the related radial groove, it withstands by friction the centrifugal force to which it is subjected.
This solution, which has long been adopted in the field of single-cylinder circular knitting machine for manufacturing socks and stockings, has some problems particularly in modern machines, which can reach high speed rotation.
In fact, in order to overcome the high centrifugal forces which act on the hooks it is necessary to obtain an increasingly higher friction between the hooks and the radial grooves of the platen. This friction force, whose effect is desirable when the hooks must not be used, becomes disadvantageous when the hooks must be actuated in one direction or the other along the radial grooves. In this occasion the friction force in fact opposes the action of the cams, and the higher the friction force, the higher the force exchanged between the heels of the hooks and the hook actuation cams. Since the wear of the heels is directly correlated to said force, there is a rapid wear of the heels of the hooks, particularly in machines operating at high speed, which necessitates frequent maintenance interventions in order to replace the hooks.
Furthermore, the friction force which acts on the hooks generates a negative torque on the platen which compels one to oversize the rotational connection between the needle cylinder and the platen in order to avoid accidental displacements between the platen and the needle cylinder, which are particularly dangerous during the extraction of the hooks, since they would cause the breakage or damage of the needles and hooks.
The friction force, which is intentionally increased by bending the hooks or with equivalent contrivances, has so far necessitated the adoption of high-strength mechanical connections between the needle cylinder and the platen and has discouraged the adoption of other types of connection.